Up to now, Patent Literature 1 has proposed a technique in which an acceleration sensor is embedded in a rear surface of a tire tread, and a tire air pressure is estimated on the basis of a detection signal of the acceleration sensor. Specifically, when the acceleration sensor using a piezoresistive effect is embedded in the rear surface of the tire tread, the detection signal reaches vibration peaks at timing when a portion of the tire tread corresponding to a placement position of the acceleration sensor contacts the ground as the tire rotates, and at a timing when the portion is no longer contacting the ground (hereinafter the timing when the tire contacts the ground is called “ground contact start time”, and the timing when the tire is no longer contacting the ground is called “ground contact end time”). For that reason, up to now, a ground contact length corresponding to a length of a tire ground contact surface in a tire traveling direction is calculated on the basis of an interval between the two vibration peaks generated at the ground contact start time and the ground contact end time, and a vehicle speed. Then, the tire air pressure is estimated on the basis of the ground contact length.